Volcanic rocks

VOLCANIC ROCKS (EN: volcanic rock; DE: vulkanisches Gesteine; FR: roches volcaniques; ES: rocas volcanicas; RU: вулканические горные породы) are the rocks, which are formed as the result of the volcanic eruptions.

Depending on the character of the eruption (the outpourings of the lavas, or the explosive eruptions), there are formed 2 types of the rocks: the outpoured, or effusive rocks, and the volcanogenic-clastic, or pyroclastic rocks; the latter rocks are partitioned into loose (the volcanic ash, sand, bombs, and others), compacted, and cemented (the tuffs, tuff breccias, and others). Besides these rocks, they distinguish the intermediate types of the volcanic rocks, namely, the tuff lavas, which have emerged as the result of the eruptions of the foaming lava flows, which are rich in the gas, and the ignimbrites, which are representing by themselves the sintered volcanogenic-clastic material, mainly acidic, with which there are covered the huge areas, which are measured in the hundreds and thousands of square kilometres. The shape of the effusive bodies is determined according to the viscosity of the lavas, and according to their temperature regime. The covers and flows are characteristical for the low-viscosity basaltic lavas, but there may be encountered also the acidic (of the liparite type) flows. The domes and needles emerge during the eruptions of the viscous lavas (dacites, liparites). The dikes and necks represent by themselves the fillings with the melt within the cracks and supplying channels. The effusive and pyroclastic volcanic rocks may be embedded in the form of the stratified strata; they exist within the cross-sections of the volcanic regions, while interbedding with the sedimentary rocks.

The volcanic rocks differ according to the chemical composition, the structural-textural peculiarities, and the degree of the preservation of the substance of the rocks. According to the chemical composition, the effusive volcanic rocks are divided into the alkaline-earth and alkaline rocks, and, besides this division, they are divided into the basic rocks (undersaturated with the silicic acid), intermediate rocks (rich in the silicic acid), and acidic rocks (supersaturated with the silicic acid). The degree of the crystallization of the lavas, and also the structures and textures of these lavas, depend on the viscosity of the melt, and on the character of its cooling. The internal parts of the effusive bodies are usually crystallized, while the external parts are scoriaceous, porous, and glassy. For the effusive rocks, there are characteristical the porphyritic, microlitic, and semi-glassy structures, and the fluidal banded, massive, and porous textures.

The effusive rocks, which have been deeply changed, and which are usually more ancient, are called the paleotypal rocks, while the unaltered rocks are called the cenotypal rocks. The most distributed cenotypal rocks are the basalts, andesites, trachytes, liparites, while their palaeotypal analogues according to the chemical composition are, respectively, the diabases, the porphyrites of the basalt type, and of the andesite type, and the porphyries of the trachyte type, and of the liparite type. There belong to the clastic volcanic rocks, along with the pyroclastic rocks (tuffs, volcanic breccias), also the volcanogenic-sedimentary rocks.

The volcanic rocks are used as the building and facing stone, and serve as the material for the stone casting (basalt, and others). The kaolinized acidic and alkaline volcanic rocks are used as the "porcelain stone" within the ceramic industry. Certain types of the volcanic ashes and tuffs (trass and pozzolans), which are possessing the binding properties, are used as the additives for the cement materials. The volcanic pumice is used as the abrasive material, and is consumed for the production of the pumice concrete. The perlite is used as the lightweight sound insulating and heat insulating fillers within the concrete, plastering, and other mixtures. Within the USSR, the large deposits of the volcanic rocks are known within the Caucasus region, within the Transcarpathia region, within the Tien Shan and Pamir regions, within the Transbaikalia region, and within the Far East and Primorye regions.